Electroacoustic transducer



April 24, 1951 DE BOER ET AL 2,549,963

ELECTROACOUSTIC TRANSDUCER Filed Jan. 11, 1947 JAN DE BOER & GERRIT SCHENKEL INVENTORS AGENT Patented Apr. 24, 1951 ELECTROACOUSTIC TRANSDUCER Jan de Boer and Gerrit Schenkel, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as

trustee Application January 11, 1947, Serial No. 721,636 In the Netherlands December 3, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires December 3, 1965 8 Claims.

The invention relates to an oscillatory system, more particularly an electrodynamic system, for example for microphones, loudspeakers, pickups, and so forth, comprising a diaphragm which, together with the air cushion located between this diaphragm and the magnet system and through one or more apertures located behind this diaphragm and filled with damping material, is acoustically coupled with a space located behind the apertures.

It is well known that systems of this kind permit of obtaining smoothing of the resonance frequency of the system.

According to the invention, the damping material forms channels parallel to the air motion occurring in the apertures during operation. This ensures damping of the system resonance lying in the range of the lower frequencies, which is more efficient than that hitherto obtainable by the conventional damping methods. This effect is probably due to the fact that with the arrangement of the channels as described, there is an air resistance which is larger in value than in.

the case of the channels referred to being arranged otherwise, other things being equal. In addition, the mass of the system is enlarged, as is known per se, by a proper choice of the total section of the channels, so that in addition to more efiicient damping, an extension of the frequency characteristic curve towards the region of the lowest frequency ensues.

The channels may, for example, be constituted by spaces between bars coaxially arranged in the aperture. It has been found that, for example, metal bars give highly satisfactory results. This is probably due to the fact that frictional heat provoked by the air motion in the channels is conducted away more efficiently by metal bars. and this acts on the resistance to the air in these channels.

The invention is particularly suitable for use in electrodynamic oscillatory systems provided with a central pole core, since use can be made with advantage of the presence of this core to arrange the said apertures in a central bore in this core. D

According to one form of construction, the apertures filled with damping material communicate with a space sealed from the open air, so that a so-called pressure microphone is obtained.

According to a further form of construction, the said apertures communicate directly with the open air, so that a so-called pressure-gradient microphone is formed.

In order that the invention may be clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawing, in which some forms of construction are represented by way of example.

Fig. 1 shows diagrammatically an electrodynamic pressure microphone, of which the bore in the core of the system is filled with coaxially arranged bars. The particular form of the air channels may be seen from section A--A in Fig. la. V

Fig. 2 shows the frequency characteristic curve and Fig. 3 the electrical equivalent circuit of the pressure microphone shown in Fig. 1.

Fig. 4 represents diagrammatically an electrodynamic pressure-gradient microphone, in which the bore in the core of the system is filled with coaxial bars.

Fig. 5 shows the electrical equivalent circuit of the microphone shown in Fig. 4.

In the electrodynamic pressure microphone shown in Fig. 1, a coil I fastened to a diaphragm 2 oscillates under the influence of the sound pressure in the magnetic field in an air gap 3 formed by a pole core 4 and a pole plate 5. The field is produced by an annular magnet 6. A space 1 located directly behind the diaphragm serves as an air cushion between the diaphragm 2 and the core 4 and communicates with a space H] by means of the air gap 3 and an annular gap 8 formed by the pole plate 5 and a ring 9 of nonmagnetic material. According to the invention, core 4 is provided with a bore II which is filled with coaxial bars [2, so that channels l3 are formed which connect the space 1 to a space 14. As an alternative, the channels concerned may be formed in a simple manner by axially inserting, in the said bore a ribbon of corrugated metal coiled so as to form a cylinder, the diameter of which fits the bore.

Particular dimensioning of spaces 1 and I0 permits of giving in known manner a smoothed variation to the frequency characteristic curve for frequencies near to about 600 c./s. Owing to resonance between the mass of air in the annular gap 8 and the rigidity of the air in the space It! it will thus be possible in the case under consideration to ensure a resonance peak for the intermediate frequency which is damped by the resistance of the air in 8, as is designated IS in Figure 2, the frequency characteristic curve of the system and owing to which the minimum 16 in the curve is filled. Space 1 serves to bring about resonance in known manner in the case of sound oscillations of the highest frequency be- 3 tween the air rigidity in I and the masses of the diaphragm 2 and the air in II altogether, which is designated I! in Fig. 2.

Space M and the air channels 13 provided, according to the invention, in the central bore ll of the magnet core 4 are critical for the reproduction of oscillations having frequencies less than 600 c./s. It will be seen from Fig. la how these channels ars formed. As soon as the system formed by diaphragm 2, air channels 13 and space M is caused to be in resonance, both the mass of the diaphragm and the mass of the air in the narrow channels are added up, as are also the rigidity of the edge of the diaphragm and the rigidity of the air in [4, owing to which a resonance in the range of the lowest frequencies to be reproduced is produced which, however, is damped by the heavy resistance to the moving mass of air in the narrow channels; this is designated l8 in Fig. 2.

It has thus been found possible to obtain, in a manner capable of reproduction, a system resonance which is below 200 c./s.

Fig. 3 shows an equivalent electrical circuit of the system described, the masses, rigidities and resistances being indicated in known manner as inductances, capacities and resistances. Connected in series with the mass l9 and the rigidity of diaphragm 2 are successively:

(a) the rigidity 2| of the air in space "i;

(b) the mass 22 of the air in the annular gap 8, the resistance 23 to the air moving in the said gap and the rigidity 24 of the air in space it;

(c) the mass 25 and the resistance 26 of the air in the channels and the rigidity 2: of the air in space l4.

It is directly obvious from the circuit-diagram that with lower frequencies the speed of the air motion in the main branch comprising mass l8 and rigidity 2B is increased by resonance in branch At these frequencies branches 28 and 29 respectively have high impedances. The speed, is, however, reduced by the air friction in the narrow channels (resistance 26).

Fig. 4 represents an electrodynamic pressuregradient microphone," whose equivalent circuit is shown in Fig. 5. Space 3| behind diaphragm 32 communicates directly with the open air through channels 33 between bars 35 coaxially arranged in the bore 3:1 of core 35. Space 3'! is fully closed by a ring 38 of non-magnetic material. The coil 39 fastened to diaphragm 32 oscillates in the air gap between pole core 35 and pole plate 40. The field is produced by an annular magnet 4|.

With higher frequencies of the sound oscillations rigidity d2 (Fig. 5) of the air in 31 together with rigidity 53 and mass M of diaphragm 32 will be caused to be in resonance, whereas with sound oscillations of lower frequencies, below 600 c./s., space 3! forms a high acoustical imped ance for the air motion in channels 33, with the result that mass M of diaphragm 32 increased by the mass of the air 45 in channels 33 and the rigidity 43 of the diaphragm is caused to be in resonance, so that a resonance peak displaced towards the lowest frequencies is pro duced which is damped by the heavy resistance 45 to the moving air mass in the said channels.

The invention may successfully be used not only for microphones and loudspeakers but for any kind of oscillatory system, the resonance frequency of which should be damped and also shifted towards the lower frequencies with respect to the case in which the measures according to the invention are not used.

What we claim is:

1. An electrodynamic transducer comprising a magnetic structure defining an air gap and including a center pole, and a vibratory system including a diaphragm and a coil attached thereto and surrounding said center pole within said air gap to define an air cushion between said diaphragm and one end of said center pole, said center pole having a plurality of longitudinal channels therein communicating between said air cushion and an air space at the other end of said pole, whereby said diaphragm is acoustically coupled through said air cushion and the air contained within said channels to said air space, the total sectional area of said channels 4 having a value at which the mass of the air in the channels and the air friction of said channels imparts a predetermined acoustic resonance characteristic to said vibratory system.

2. A transducer as set forth in claim 1 further including means secured to said magnetic structure to enclose the air space at the other end of said pole piece.

3. An electrodynamic transducer comprising a magnetic structure defining an air gap and including a center pole, and a vibratory system including a diaphragm and a coil attached thereto and surrounding said center pole within said air gap to define an air cushion between said diaphragm and one end of said center pole, said center pole having a bore therein and a plurality of rods in parallel juxtaposition arranged within said core to define a plurality of relatively narrow longitudinal channels cornrnunieating between said air cushion and an air space at the other end of said pole, whereby said diaphragm is acoustically coupled through said air cushion and the air contained within said channels to said air space, the total sectional area of said channels having a value at which the mass of the air in the channels and the air friction of said channels imparts a predetermined acoustic resonance characteristic to said vibratory system.

4. An electrodynarnic transducer comprising a reentrant magnetic structure including a center pole and a plate pole arranged to define a magnetic flux air gap, a vibratory system includin a diaphragm and a coil attached thereto and surrounding said center pole within said air gap to define an air cushion, and a nonmagnetic annular member supported by said center pole below said plate pole to define an annular air gap communicating between said magnetic flux air gap and the air chamber formed within said magnetic structure, said center pole having a plurality of longitudinal channels therein communicating between said air cushion and an air space at the other end of said poles whereby said diaphragm is acoustically coupled through said air cushion and the air contained within said channels to said air space, the total sectional area of said channels having a value at which the mass of air in the channels and the air friction of said channels imparts a predetermined acoustic resonance characteristic to said vibratory system.

5. An electrodynamic transducer as set forth in claim 4 further including a cuphaped member secured to the end of said reentrant magnetic structure opposing said plate pole for enclosing said air space.

6. An electrodynamic transducer comprising. a

reentrant magnetic structure including a center pole and a plate pole arranged to define a magnetic flux air gap, a vibratory system including a diaphragm and a coil attached thereto and surrounding said center pole within said air gap to define an air cushion, and a nonmagnetic annular member supported by said center pole below said plate pole and peripherally connected to said plate pole to form an enclosed air chamber within said magnetic structure, said center pole having a plurality of longitudinal channels therein communicating between said air cushion and an air space at the other end of said poles whereby said diaphragm is acoustically coupled through said air cushion and the air contained within said channels to said air space, the total sectional area of said channels having a value at which the mass of air in the channels and the air friction of said channels imparts a predetermined acoustic resonance characteristic to said vibratory system.

7. An acoustic transducer comprising a magnetic structure including a pole member, and a vibratory system including a diaphragm adjacent one end of said pole member to define an air cushion therebetween, said pole member having a plurality of longitudinal channels therein communicating between said air cushion and an air space at the other end of said pole member,

whereby said diaphragm is acoustically coupled through said air cushion and the air contained within said channels to said air space.

8. An acoustic transducer comprising a magnetic structure including a pole member, and a vibratory system including a diaphragm adjacent one end of said pole member to define an air cushion therebetween, said pole member having a bore therein and a plurality of rods disposed in parallel juxtaposition forming a plurality of relatively narrow longitudinal channels communicating between said cushion and an air space at the other end of said pole member, whereby said diaphragm is acoustically coupled through said cushion and the air in said channels to said air space.

JAN DE BOER.

GERRIT SCHENKEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,634,380 Nayler July 5, 1927 1,957,765 Finch May 8, 1934 1,964,606 Thuras June 26, 1934 2,197,649 Neuschatz Apr. 16, 1940 2,295,483 Knowles Sept. 2, 1942 2,312,238 Cunningham Feb. 23, 1943 2,367,026 Hutter Jan. 9, 1945 2,395,166 Collins Feb. 19, 1946 2,429,470 Knowles Oct. 21, 1947 OTHER REFERENCES Ser. No. 409,712, Gorike (A. P. C.), published May 18, 1943. 

